Colocasia esculenta is a fast-growing herbaceous plant that originates from a large corm and can grow to 4 ft. (1.5 m) in height. It has been intentionally introduced in many tropical and subtropical regions to...

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Summary of Invasiveness

Colocasia esculenta is a fast-growing herbaceous plant that originates from a large corm and can grow to 4 ft. (1.5 m) in height. It has been intentionally introduced in many tropical and subtropical regions to be used as a food crop and animal fodder (Onwueme, 1999; Safo-Kantaka, 2004), and has subsequently escaped from cultivated areas into natural areas where it becomes invasive (Langeland et al., 2008). In Australia, it is regarded as an environmental weed in Queensland, New South Wales and south-western Western Australia (Queensland Department of Primary Industries and Fisheries, 2011). In PIER (2012) it is listed as invasive in Hawaii, French Polynesia, the Juan Fernández Islands, the Galapagos, the Kermadec Islands and the Marshall Islands, although it is also widely cultivated in some of these and other Pacific Islands. It is also viewed as an invasive species or aggressive weed in parts of the Caribbean and Americas (see Distribution Table for details).

C. esculenta has several adaptations that aid its survival as a weed. It has the ability to reproduce both sexually by seeds and vegetatively by corms, tubers, and root suckers, and it is adapted to grow in a great variety of substrates and habitats ranging from full sun to deep shaded areas (Safo-Kantaka, 2004). As a highly variable species, invasiveness varies with cultivar: in Australia, for example, the purple-stalked ornamental garden cultivars (i.e. 'Euchlora' and esculenta 'Fontanesii') are the most invasive (Queensland Department of Primary Industries and Fisheries, 2011).

Notes on Taxonomy and Nomenclature

The family Araceae comprises about 117 genera and 4095 species distributed mostly in tropical areas in the New World, but also in Australia, Africa, and north temperate regions (Stevens, 2012). The genus Colocasia includes 8-16 species native to tropical Asia. The species C. esculenta, also known as taro or cocoyam, is cultivated and naturalized throughout the tropics (Acevedo-Rodríguez and Strong, 2005; Randall, 2012). Many cultivars adapted to saline conditions, low water availability and/or seasonal flooded soils have been created and are also widely cultivated throughout tropical and subtropical regions (Onwueme, 1999). The taxonomy of Colocasia cultivars with edible corms is confusing. The number of species recognized varies among authors, some recognizing C. esculenta as a highly variable species, others recognizing several species or varieties within this complex [see also ‘Other Botanical Information’ in the Biology and Ecology section]. Some forms are cultivated for their edible corms while others are grown for their ornamental foliage. In the early twentieth century around 300 varietal names for the crop were recorded in Hawaii, about half of which were estimated to be synonyms (Whitney et al., 1939).

Description

Perennial, glabrous, herb growing to a height of 1 m or more, with a massive, fleshy corm at the base, and lateral, thick, edible runners. Root system adventitious, fibrous, and shallow. Storage stem (corm) massive (up to 4 kg), cylindrical or spherical, up to 30 x 15 cm, usually brown, with lateral buds located above leaf scars giving rise to new cormels, suckers or stolons. Leaves are arranged in a loose rosette; blades pointing downward, 23-55 × 12-38 cm, cordate or lanceolate, sub-coriaceous, green above, glaucous below, the apex obtuse, acute or shortly acuminate, the base peltate-cordate, the margins more or less wavy, with a submarginal collecting vein; petioles erect, to 85 cm long, inserted 3-7 cm from base of blade. Inflorescences axillary, ascending, solitary; peduncles nearly as long as the petiole, cylindrical; spathe fleshy, to 35 cm long, the tube green, the blade lanceolate, not much wider than the tube, yellow to orange, flexing open near the base, then deflexing and dropped; spadix yellow, much shorter than the spathe, the sterile flower zone and the distal appendage shorter than the fertile zones. Fruit is a many-seeded berry, densely packed and forming a fruiting head. Seeds are ovoid to ellipsoid, less than 2 mm long, with copious endosperm (Acevedo-Rodríguez and Strong, 2005; Langeland et al., 2008).

Plant Type

Distribution

C. esculenta is native to tropical Asia (Swimming Shoes Shorts Black Dc Breakwall Volley q1TwwHS; USDA-ARS, 2012), originating in South-East or southern Central Asia, where it was probably cultivated before rice. It has been actively cultivated throughout tropical and subtropical regions. Today it is grown throughout the West Indies and in West and North Africa. In Asia, it is widely planted in south and central China and is grown to a lesser extent in India. It is now a staple food in many islands of the Pacific including Papua New Guinea, where it has prestigious as well as economic value, playing an important role in traditional gift-giving and ceremonies. It also has great cultural importance in Hawaii, where growing the crop was not merely an activity of food production but was strongly bound to the people’s culture and beliefs about creation (Cho et al., 2007).

In Indonesia, C. esculenta is a staple food on the Mentawai Islands and for Melanesians in Irian Java. It is cultivated to a lesser extent in Bogor and Malang in Java, and on Bali. In Malaysia, it has been used for more than 2000 years and is now found throughout the country. C. esculenta is grown throughout the Philippines but is most important in eastern and central Visayas and the Mindanao and Bikol regions.

In Australia, C. esculenta is naturalised in northern and south-eastern Queensland, in south-western Western Australia, and in the coastal districts of central and northern New South Wales. It is also naturalized on Lord Howe Island, Norfolk Island and Christmas Island (Queensland Department of Primary Industries and Fisheries, 2011).

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

C. esculenta is thought to have been domesticated in northern India, but independent domestication in New Guinea has also been suggested (Safo-Kantaka, 2004). Domestication is believed to have taken place at a very early date, even before the domestication of rice. Archaeological evidence suggests human use of the plants 28,000 years ago in the Solomon Islands (Loy et al., 1992). It was spread by human settlers eastward to New Guinea and the Pacific over 2000 years ago, where it became one of the most important food plants economically and culturally. It was one of the first crops introduced to Hawaii by Polynesians around 900 to 1000 AD, most likely in multiple arrivals (Cho et al., 2007).

Distribution to China and via Arabia to Egypt and East Africa also occurred at least 2000 years ago. From there C. esculenta was introduced by Arab people to West Africa. It was introduced into Europe from Egypt (Greenwell, 1947).

During the seventeenth century, C. esculenta was introduced from Africa to the Americas as a food crop for slaves by Spaniard, Portuguese, and British slave traders (Greenwell, 1947). For example, as early as 1647, taro was cultivated in Barbados as a slave dietary staple (Carney and Rosomoff, 2009). Later, by 1864, it was reported as “naturalized” in Jamaica and St. Kitts and widely cultivated in most islands in the West Indies (Grisebach, 1864). In Puerto Rico, the first report of this species was made by Bello in 1883. In the southeastern United States, it was introduced in 1910 by the Department of Agriculture as a substitute crop for potatoes (Greenwell, 1947; Langeland et al., 2008).

Risk of Introduction

The risk of introduction of C. esculenta is high, especially in areas near to cultivated fields. It is an aggressive weed that has escaped from cultivated areas and become invasive in tropical and subtropical regions of the world (Randall, 2012). Plants produce underground corms and stems that are commercialized as food for human consumption and animal fodder. It is also exploited as an ornamental (Onwueme, 1999; Safo-Kantaka, 2004). C. esculenta is a fast-growing plant that can be dispersed by seed and by corms which re-sprout easily forming new plants.

In Puerto Rico, this species escaped from cultivation and persists along riverbanks and in moist forest understory in Arecibo, Caguas, Carolina, Jayuya, Loíza, Salinas, San Juan, and Toa Baja (Acevedo-Rodríguez and Strong, 2005). In Florida, this weed was widely naturalized along streams, marshy shores, canals, and ditches in more than 235 public water bodies by 1994 (Langeland et al., 2008).

In Australia, C. esculenta is an environmental weed invading waterways and wetlands and replacing native aquatic plants. It is listed among the 200 most invasive plants in the region of Queensland and is also a problem in the coastal districts of New South Wales and along the waterways in Western Australia (Queensland Department of Primary Industries and Fisheries, 2011).

Species Vectored

Biology and Ecology

Chromosome numbers reported for C. esculenta include 2n = 22, 26, 28, 38 and 42 (Coates et al., 1988; Sreekumari and Mathew, 1989; Onwueme, 1999). The disparity in the numbers may be due to the fact that chromosomes in this species are prone to unpredictable behaviour during cell division. The most commonly reported results are 2n = 28 or 42 (Coates et al., 1988).

Reproductive Biology

Under natural conditions, reproductive activity in C. esculenta occurs only occasionally. However, flowering can be artificially promoted by application of gibberellic acid. The inflorescence arises from the leaf axils, or from the centre of the cluster of unexpanded leaves. Each plant may bear more than one inflorescence. The spadix is 6-14 cm long, with female flowers at the base, male flowers towards the tip, and sterile flowers in between, in the region compressed by the neck of the spathe. Pollination in this species is probably performed by flies (Onwueme, 1999).

Physiology and Phenology

Information on growth is mainly available for C. esculenta grown as a crop. Growth of leaves on main plants is slow during establishment, but is rapid from 1.5-2 months after planting, with most rapid leaf growth between 3 and 5 months after planting. During the fourth or fifth month, leaf size, leaf dry weight, leaf area, leaf area index and plant height reach their maximum values. Leaf number varies and there is a continuous turnover of leaves. After peaking, leaves become smaller with shorter petioles and leaf number decreases.

Only a portion of corm or stem is needed to establish new plants. Main corm growth begins as early as 2 weeks after planting, with rapid corm growth beginning two months after planting under rainfed conditions and 3-5 months after planting under irrigated conditions. Corms reach maximum weight at 10-11.5 months when rainfed and 12-15 months when irrigated, but are usually harvested before this time.

Sucker growth generally begins 2.5 months after planting. The number of suckers depends on cultivar and management.

Corms can remain underground and survive through unfavourable environmental conditions (i.e., drought). If they are not harvested, corms will sprout and give rise to new plants at the onset of the next favourable season. Under favourable environmental conditions, plants may continue growth for several years (Onwueme, 1999; Safo-Kantaka, 2004).

Environmental Requirements

C. esculenta is best suited to tropical lowland areas with annual precipitation greater than 2000 mm, and evenly distributed, although there are many upland varieties with water requirement of much less than 2000 mm. It is well adapted to high temperatures (20-35°C) and shaded conditions and for that reason it is common to find it growing under coconut, cocoa or coffee plantations: when cultivated, it is often grown as an intercrop with such tree crops. It also grows well in wetlands including paddies with a continuous supply of flowing water, furrow-irrigated fields, and raised beds in poorly drained swamps. As a crop, many cultivars adapted to wet soil conditions, drought, partially saline conditions, and low temperatures have been produced. In Papua New Guinea, it is as frost hardy as sweet potato.

C. esculenta grows best in well-drained loamy soils, but it has the potential to grow in a wide variety of soils including clay, sandy, and loamy soils with pH normally ranging from 5.5 to 6.5 (Onwueme, 1999; Safo-Kantaka, 2004). In Malaysia it is reported to tolerate soil pH ranging from 4.2 to 7.5. Good crop yields require high fertility.

Other Botanical Information

When cultivated as a crop, there are two forms of C. esculenta or taro. The dasheen type has a large central corm with a few small cormels which are generally not eaten. The eddoe type produces a smaller central corm surrounded by large, well-developed cormels which are the main harvestable yield. Eddoes are often more drought-hardy than dasheens.

Although the eddoe type is frequently classified as a separate species, C. antiquorum Schott, it is more generally accepted that it is a variety, C. esculenta var. antiquorum (Schott) Hubb. & Rehder, of a very variable species that includes both dasheens and eddoes.

There are many taro cultivars, and these are distinguished by morphological characteristics (e.g., corm size and shape) as well as time taken to mature. Colour of corm flesh, lateral buds, petioles, and leaf blades are also used to differentiate cultivars.

Means of Movement and Dispersal

C. esculenta spreads by seeds, underground corms, tubers, root suckers, and stems. Seeds in this species are uncommon. Fragments of corms and tubers can be easily dispersed by streams and floods (Wagner et al., 1999; Langeland et al., 2008). Tubers, corms, and root suckers easily re-spread producing new plants. In addition, corms can remain on the ground for several months waiting for suitable environmental conditions to sprout (Onwueme, 1999). Corms and tubers can also be dispersed by movement of soil by vehicles and farming machinery.

Uses

C. esculenta, commonly known in its crop form as taro, is mainly cultivated for its starchy stem tubers and corms. These corms are eaten boiled, fried or roasted as a side dish or are used to produce starch and flour. Taro corm puree makes an easily digested, low-allergenic baby food. In times of scarcity, this species is used as a famine food and in some regions leaves are used for food after cooking.

Waste leaves, corms and peel can be cooked or fermented into silage for animal feed. Most taro in South-East Asia is consumed by humans, but it also has uses in religious festivals and in folk medicines and is fed to livestock, primarily pigs.

In areas of Indonesia where rice is not grown, taro is eaten as a staple, baked, boiled or cooked in bamboo tubes. In Java, confections are prepared from taro flavoured with coconut and sugar: fried taro slices and taro chips are popular snacks. The leaves are used in preparing 'buntil' (salted fish with spices, grated coconut and vegetables, wrapped up and steamed in a taro leaf), and petioles are cooked. In Malaysia, taro is cooked in similar ways and also plays a role in religious festivals. Leaves are boiled and eaten as salad with spicy sauce, and petioles are cooked with coconut milk, meat and prawns. Taro in the Philippines is used primarily when more popular starches and green vegetables are in short supply. Corms are boiled, chipped and fried or made into confections. In Hawaii and parts of Polynesia, the corms are cooked and pounded into a paste that is allowed to ferment to produce 'poi'. A steamed pudding is made from grated taro and coconut.

Fibre obtained from the leaf stalk has been used for plaiting. In Asia and Africa, this species is also used in traditional medicine to treat arterial hypertension, liver problems, ulcers, snakebites, and rheumatism (Onwueme, 1999; Safo-Kantaka, 2004).

Materials

Medicinal, pharmaceutical

Similarities to Other Species/Conditions

C. esculenta may be confused with other plants having large arrowhead-shaped leaf blades, such as species in the genera Alocasia and Xanthosoma (i.e., Xanthosoma sagittifolium). However, leaves of all similar-looking species are not peltate (i.e., their petioles are attached at the leaf-blade margin, rather than in the middle as with C. esculenta; Langeland et al., 2008).

Prevention and Control

Mechanical removal of C. esculenta may be effective but is labour intensive. All corms and tubers should be removed to prevent spread. Dense stands should be removed using specialized machinery.

Chemical Control

Chemical control of C. esculenta should include mechanical cutting of the plants to the base followed by applications of N-phosphonomethyl-glycine (glyphosate). Follow-up treatments are recommended at 6 weeks intervals until control is completed (Queensland Department of Primary Industries and Fisheries, 2011). This procedure is not recommended when plants are growing near rivers or streams.